1. Field of the Invention
The present invention is directed to a DC-powered electric blanket having a PWM-based control circuit. The invention is further directed to such a DC-powered electric blanket utilizing a PWM control circuit having battery conservation features and an associated method of controlling such an electric blanket.
2. Description of Related Art
It is known to adjust the output power of a battery providing power to a device, such as a spotlight, table lamp or other such source of light. Considering, for example, the context of lighting devices, one known circuit incorporates pulse width modulation (PWM) to automatically increase the duty cycle of the signal that provides power to the lamp as the voltage of the battery decreases, to thereby maintain a constant power supply and light intensity. It is also known to manually decrease the duty cycle to reduce the intensity of the light as the battery voltage decreases. Examples of such control circuits are described in U.S. Pat. No. 4,499,525 to Mallory and in U.S. Pat. No. 6,040,660 to Schmidt et al., which are incorporated herein by reference. Note that, in the case of the former, the light intensity is maintained at the expense of battery conservation.
Pulse-width modulation (PWM) based control techniques are known in heating devices, as well. For example, U.S. Pat. No. 4,950,868 to Moss et al. and U.S. Pat. No. 5,023,430 to Brekkestran et al. describe electrically-heated clothing using PWM-based control circuits.
U.S. Pat. No. 6,122,162 to Keane discloses an electric heating blanket having a control circuit that monitors resistivity of a heating element as a basis for control. While Keane suggests control mechanisms that vaguely resemble PWM-based control, they are not true PWM-based control mechanisms.
Co-pending U.S. patent application Ser. No. 10/277,087, entitled, xe2x80x9cPWM Controller with Automatic Low Battery Reduction Circuit and Lighting Device Incorporating the Controller,xe2x80x9d filed Oct. 22, 2002, commonly assigned and incorporated by reference herein in its entirety, presents a PWM-based control circuit and a lighting device using the circuit. The PWM-based control circuit disclosed therein includes a low voltage detection circuit that permits the control circuit to detect when the voltage of a power supply goes below a predetermined level. In such circumstances, the control circuit decreases the duty cycle of a PWM control signal in order to conserve power. Manual adjustment of duty cycle is also enabled.
It would be useful if there were a DC-operated heating blanket that incorporated a PWM-based controller, especially one that includes such battery saving features.
It is an object of the invention to provide a means by which to permit a user to manually adjust the intensity of an electric heating blanket, and which automatically reduces power drawn from a power source to the device as the power possessed by the power source decreases.
It is a further object of the invention to provide a means for varying the heat intensity of a DC-powered electric heating blanket that can be manually adjusted and a means for automatically reducing the power drawn from the DC power source by the blanket as the voltage of the power source decreases.
It is yet a further object of the invention to provide a DC-powered electric heating blanket utilizing PWM-based control.
The above and other objects are accomplished in accordance with the invention by the provision of a PWM-based controller for controlling a voltage provided from a power source to a DC-operated electric heating blanket. The controller comprises a square wave producing circuit that produces a variable-duty-cycle square wave for applying voltage to the blanket. A voltage varying circuit is included for producing a selectively variable voltage that is fed to a control input of the square wave producing circuit for controlling a duty cycle of the variable-duty-cycle square wave. The controller further includes a low voltage detection circuit that monitors the power source and is also coupled to the control input of the square wave producing circuit; the low voltage detection circuit automatically produces a voltage that decreases the duty cycle of the variable-duty-cycle square wave when the voltage of the power source decreases below a predetermined level.
The above and other objects are also accomplished in accordance with the invention by the provision of a PWM-based method for controlling a voltage provided from a power source to a DC-operated electric heating blanket. The method comprises step of generating a variable-duty-cycle square wave for applying voltage to the blanket. The method further includes the step of controlling a duty cycle of the variable-duty-cycle square wave. The method further includes steps of detecting a low voltage condition of the power source and using the result to control the duty cycle of the variable-duty-cycle square wave. More specifically, as the detected voltage decreases below a predetermined level, the duty cycle of the variable-duty-cycle square wave is decreased.